This short paper compares and contrasts two different ideas of the origins of Cetaceans. Cetaceans are defined as ‘The order of marine mammals that comprises the whales, dolphins, and porpoises. They have a streamlined hairless body, no hindlimbs, a horizontal fin, and a blowhole on top of the head for breathing’ (“Cetacea | Definition of Cetacea in English by Oxford Dictionaries”, n.d.). Upon comparing the two the conclusion drawn, is that all Cetaceans evolved from its ancestor Indohyus (Fig. 1) through around 20 million of years of evolution and adaptations.
The very first origins of life are debated by theologians, scientists and cultural denominations across the globe. There are many theories that emerged in ancient times as seen in Greek mythology and Australian Aboriginal dreamtime (refs). Many people also believe that life was created by a Creator and Christians call this divine being, God. There are also several theories of evolution that have been proposed since the 1850’s (ref). Evolution and creationism will be investigated to explain the origins of the Cetaceans: whales, dolphins and porpoises.
Described by Charles Darwin as “Descent with modification” in his work “On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life” (Darwin, 2008), the theory of natural selection sums up the idea of evolution. The theory of evolution suggests that complex creatures evolve from more simplistic ancestors naturally over time (Mancia, 2018). This can be explained due to random mutations that occur in the genetic code at any point in time, over many generations of offspring. Darwin’s thesis of natural selection says that all advantageous mutations will survive while the ‘genetically inferior’ will die off (ref). Creationism provides a different perspective on the origins of life.
Creationism posits that……
By understanding that the Bible, the foundation of Christianity, was written thousands of years ago at a time where lifestyle and knowledge of basic processes was far less than now, then Creation theory is plausible. However, so is evolution. It is possible that God created the universe and all that it entails in a matter of six days. But we must question the idea of six days. It could potentially be a sort of placeholder to make it more comprehensible to humans. It could be that 6 days is not 144 hours as we believe it is today. It may instead mean millions of years. Creationism and evolution do not need to be viewed as two distinct sides of a coin though creationism presumes that there is a God while evolution presumes that life modifies and adapts over time. While there are holes in both creationism and theories of evolution, evolution is based on natural, observable and measurable processes and therefore can explain the origin of the Cetaceans.
embryology phylogenetics?, comparative anatomy, summarize points? Double pulley ankles/fossil record(Arg. 2) Embryology (Arg. 3)
A Brief History
According to evolution theory, the phylogenetic ‘story’ of the Cetaceans began around 55 million years ago and saw whale mammalian ancestors moving from their terrestrial environment to the aquatic environment, possibly due to a higher abundance of food to be found in the ocean. This has been coined as ‘the walk back into the ocean’ (Mancia, 2018). It begins with Indohyus, a small animal no larger than the size of a cat. Indohyus belongs to the order Artiodactyl which are todays cows, giraffes, sheep, deer, pigs, and hippopotamus (Wormworth, n.d.). The first ancient whales, however, are known as Pakicetids. Pakicetus inachus (Fig. 2) was a small mammalian creature which had four legs, was small in stature and had a tail. They are believed to have had sharp teeth that also had high cusps which have been used to determine that it had a fish-like diet. It also has the involucrum present and its eyes were positioned on the top of the skull. It also had a very distinct type of astragalus that is found in Artiodactyls, called a double pulley ankle. However, recently it has been discovered that there was a transitional form between Indohyus and Pakicetus known as Rodhocetus asrani (Fig. 3). Rodhocetus is a few million years before Pakicetus.
Next in line was Ambulocetus Natans (Fig. 4). This almost completely aquatic creature was much larger than Pakicetus and lived around 49 million years ago. Its eyes were still positioned high on the skull, it also had ears that were similar to that of a modern whale.
In order to keep the history as brief as possible there will be a few transitional forms skipped and on to the next major few. The forms skipped include (not chronological): Kutchicetus, Aetiocetus, Eurinodelphis, Remingtonocentidae, Mammalodon, Prozeuglodon, Maiacetus innus and Artiocetus. Short descriptions of each will be provided in the Glossary (Refer to Table of Contents for page number).
The final forms before todays whales are the Dorudon atrox and Basilosaurus cetoids (Fig. 5 & Fig. 6 respectively). These large whales were carnivores and grew to large sizes. They have small hind legs that still contain the double pulley ankle and involucrum. They are much more adapted to the aquatic lifestyle and would not be able to survive on land due to the size of the hind legs and bodily mass.
See Appendix for large summary of major Odontoceti transitional forms
When comparing creationism and evolution the first thing to consider is comparative anatomy. It is known that the involucrum is a body part shared between all Cetaceans since Indohyus (Fig. 7). Inside a mammals ear there is a dome-like protrusion of bone that contains the middle of the ear, known as Tympanic bulla (plural bullae). However, in the aforementioned mammals the Tympanic bulla are more dense and thick than all other mammals, this is called the involucrum. (“Science Word of the Day: Involucrum”, 2015).
There is also evidence found that further backs the idea of evolution rather than creation. There are a large amount of transitional forms of cetaceans, fossilised missing links, that have been uncovered (Fig. 8) that show the differences in the position of the nasal apparatus over time. This transition has occurred so that the Cetacean can keep its eyes under the water to scan while they can also be at the surface to breathe.
Furthermore, unlike fish, Cetaceans like all mammals have two completely developed lungs and need to come to the surface to breathe air. They also give live birth and feed their young milk. They are also warm-blooded which is an extremely rare occurrence for all sea creatures. Evolution theory in these cases is evidenced by the presence of transitional forms and explains this phenomena.
It can be seen in baleen and sperm whales that they have reduced skeletal remnants of where once were the hindlimbs seen in Archaeocetes. As mention in the brief history there is a presence of an ankle in certain Neoceti known as a double pulley ankle. It is called this because of its ‘pulley like appearance’ (Fig. 9). This type of ankle is only found in today’s artiodactyls. This indicates that the Cetaceans common ancestor is artiodactyls.
When looking at evolution in terms of embryology there is further evidence that supports the idea that Cetaceans are successors to land mammals. It can be seen in the foetal development of dolphin’s similarities to human embryos. As seen in (Fig. 10 & 11)
Bringing a point from comparative anatomy. It can be seen in the images below that the nasal structure migrates from the front of the face like any animal and slowly moves up the skull. (Note: stage is respective to images not to actual developmental periods). In this example, the forelimb buds disappear as they are not needed by the dolphin but their presence indicates that they once had legs, supporting evolution theory rather than creation by design.
Fossil records can be used further support the idea that all Cetaceans evolved from a common ancestor. To begin, as mentioned numerous times above it can be seen that the nasal transition from the front of the snout to the top of the head is very evident. It can be summed up simply using three images (Fig. 15, Fig. 16 and Fig. 17).
The fact that there are fossils that have been dated and chronologically follow eachother and display clearly that the nasal passage has evolved to be on top of the head supports the thesis of evolution.?